In recent years, various industrial and technology sectors, including aerospace, industrial, medical, and municipal, are turning to LED based lighting systems as alternatives to existing lighting solutions. This is due, at least in part, to the fact that LEDs can offer very high luminous intensity, which can be controlled over a wide range with suitable power conversion and driver circuit. Moreover, LEDs are relatively small and relatively more reliable.
Many power supply topologies may be used to drive and control the luminous intensity of LEDs. A vast majority of the proposed topologies are powered from an AC source, and thus rely on a power factor corrected (PFC) AC-DC converter at the front end. One common PFC AC-DC converter topology that is used is the boost converter because it performs well at medium to high power levels. However, one downside of this topology is that the output voltage needs to be higher than the AC source peak voltage, thus necessitating, in most applications, another DC-DC converter, a buck converter, in order to reduce the voltage level used to drive the LEDs. Adding isolation in the downstream DC-DC converter is possible but it negatively impacts overall system efficiency, size, and weight. Hence, the non-isolated, two-stage boost and buck power supply is the preferred topology for medium power LED applications, such as aircraft applications. This, as will now be explained, can present certain drawbacks in the aircraft environment.
In aircraft electrical power systems, the AC return is tied to chassis. Thus, in the unlikely event the LED side of the above-described non-isolated power supply was to become shorted to chassis, the source finds a low impedance path through the short and the controller goes out of bounds, resulting in the power supply potentially being damaged. The conventional technique that is used to protect a non-isolated power supply is to measure both the high side (supply line) and low side (return line) current, and turn-off the buck converter gate driver when the differential current exceeds a predetermined limit. The presently used techniques are not sufficiently fast, and will not be able to protect against the ground fault condition, even with existing differential current monitoring and protection techniques, thus potentially damaging the power supply.
Hence, there is a need for a system to detect a non-isolated power supply output-to-chassis short, and protect the non-isolated power supply from the detected short to prevent damage to the power supply. The present invention addresses at least this need.